The invention relates to a method for monitoring the status of a plurality of connected battery cells in a battery pack, said method comprising: arranging said battery cells in at least two groups of cells; connecting said groups of cells to a sensor unit; and providing a measurement of at least one parameter indicative of the state of operation of said battery pack by means of said sensor unit.
The invention also relates to a battery management system for monitoring the status of a plurality of connected battery cells in a battery pack, comprising: a plurality of battery cells which are arranged in at least two groups of cells; a sensor unit connected to said groups, said sensor unit being configured for providing a measurement of at least one parameter indicative of the state of operation of said battery pack; and a control unit connected to said sensor unit.
The invention can be applied in vehicles, such as cars, trucks, buses and construction equipment. Although the invention will be described with respect to an application in the form of a bus, the invention is not restricted to this particular type of vehicle, but may be used in other vehicles.
In the field of vehicles, there is a steady increase in research and development related to propulsion of vehicles with alternative power sources, i.e. power sources being used as alternatives to conventional internal combustion engines.
An internal combustion engine, for example in the form of a gasoline engine or a diesel engine, offers high efficiency with relatively low fuel consumption. However, environmental concerns have led to an increase in development of more environmental-friendly power sources for vehicles. In particular, it can be noted that electrically operated vehicles has emerged as a promising alternative.
Today, there exist various types of vehicle propulsion systems comprising electric machines. For example, a vehicle can be operated by means of an electric machine solely, or by means of an arrangement comprising both an electric machine and an internal combustion engine. The latter alternative is often referred to as a hybrid vehicle (HEV), and can for example be utilized in a manner in which an internal combustion engine is used for operating the vehicle while driving outside urban areas whereas the electric machine can be used in urban areas or in environments in which there is a need to limit, the discharge of harmful pollutants such as oxides of nitrogen, fossil carbon dioxide and carbon monoxide.
The technology involved in electrically operated vehicles is closely related to the development of electrical energy storage systems, such as battery-related technology for vehicles. Today's electrical energy storage systems for vehicles may comprise a battery pack with a plurality of rechargeable battery cells which, together with control circuits, form a system which is arranged in a vehicle and which is configured for providing electric power to an electric machine. A hybrid vehicle is also often arranged so that the energy storage system is charged during braking, by means of a process known as regenerative braking.
A vehicle which is operated by means of an internal combustion engine and an electric machine supplied with power from a rechargeable electrical energy storage system is sometimes referred to as a plug-in hybrid electric vehicle (PHEV). A plug-in hybrid electric vehicle normally uses an energy storage system with rechargeable battery cells which can be restored into a condition involving a full charge through a connection to an external electric power supply. The external power supply can be in the form of the common electric grid power system which can be accessed via a conventional power cord, or can be in the form of other arrangements depending on the vehicles involved and the power need for the recharging process.
During charging, a high amount of energy must be fed into the energy storage system in a relatively short time in order to optimize the vehicle's range of driving.
For this reason, the actual charging of the energy storage system is suitably implemented through a process in which a control unit on the vehicle requests a charging process to be carried out by means of an external electric power supply. This is carried out after the energy storage system and the external power supply have been electrically connected by means of suitable connector elements.
With reference to the field of automotive technology, an energy storage system normally comprises a battery pack with a large number of battery cells. Using a plug-in hybrid vehicle as an example, a battery pack may for example be of the lithium-ion type. In the event that a 600 V lithium-ion battery pack is used, approximately 200 battery cells connected in series will then be needed to achieve a desired voltage in order to operate the vehicle. The available range for driving the vehicle then depends on certain parameters such as the state of charge (SOC) of the battery pack. The state of charge is an important parameter to use in order to prevent batteries from being operated during under- or over-charging situations, and to manage the energy in electric vehicles. The state of charge needs to be estimated since no direct measurement is available for this parameter.
Furthermore, it is known that batteries degrade over time, and there is a need to diagnose the decrease in performance estimated by means of battery parameters that change during the lifetime of the battery, i.e. cell capacity and ohmic resistance. These parameters can be used for health monitoring, to estimate the so-called state of health (SOH) of the battery, and prognostics, to predict when it will fail or reach its end of life.
It is also known to use a battery management system in a vehicle in order to ensure safe operating conditions of the vehicle. In such a battery management system, monitoring each cell is desired but since a battery pack could include many cells, this task could be very time consuming and may put heavy demands on a control unit as regards the capacity for calculations. There is consequently a need for accurate monitoring of the battery cells of a battery pack. In particular, it is necessary to estimate parameters such as the state of charge (SOH) and the state of health (SOH) of a battery pack.
The U.S. Pat. No. 8,054,034 discloses a battery management system configured for monitoring and controlling a battery pack with a plurality of battery cells. The document discloses that the battery cells are arranged in a number of groups and that these groups are connected to a sensing unit which senses the voltages of the battery cells. These measurements are used for determining whether there is a need for a cell balancing process of the battery pack.
Although the system according to U.S. Pat. No. 8,054,034 is arranged for monitoring a battery pack in which a number of battery cells are configured in groups and monitored, there is a general problem with previous solutions in the sense that they are relatively complex and costly with regard to battery measurements and calculations.
Consequently, it is desirable to provide a method and an arrangement which solve the problems associated with prior solutions and by means of which the status of a battery pack can be monitored in an accurate and cost-efficient manner. It is desirable to provide a method and an arrangement by means of which the state of charge (SOC) and the state of health (SOH) of a battery pack can be estimated in a cost-efficient manner.
According to a first aspect of the invention, in a method for monitoring the status of a plurality of connected battery cells in a battery pack, said method comprising: arranging said battery cells in at least two groups of cells; connecting said groups of cells to a sensor unit; and providing a measurement of at least one parameter indicative of the state of operation of said battery pack by means of said sensor unit. The method further comprises: arranging said groups of cells in a manner so that at least two of said groups comprise two or more cells and at least two of said groups overlap so that a cell forms part of said overlapping groups; and connecting said sensor unit to said groups; and wherein the number of groups is less than the number of cells.
An advantage with the present invention is that it contributes to a decreased computational workload during said monitoring. A particular advantage is that the number of required sensors can be decreased as compared with known solutions, but without affecting the detectability and isolability properties needed for fault diagnosis of the battery pack. This means that the total cost for a battery pack including the required sensor units can be greatly reduced as compared with today's solutions.
According to one embodiment, the groups of cells are arranged in a manner so that it fulfills the relationship: number of groups−number of cells/2>1. An advantage with this embodiment is that it defines that the amount of sensors associated with a battery pack can be greatly reduced as compared with prior solutions.
According to one embodiment, the sensors are used for measuring at least one parameter related to the state of operation of said battery cells, said parameter being at least one of the following: the battery current; the terminal voltage of at least one cell; and the temperature of at least one cell. This means that measurements and fault diagnosis based on said parameters can be implemented in a cost-effective manner with reduced computer workload as compared with known solutions. Also, the invention offers a possibility of calculating a “remaining useful life” property of the battery pack, i.e. by using the capacity or resistance estimates.
Furthermore, according to one embodiment, the method comprises estimating, based on the measured parameter, at least one of the following properties of each battery cell or groups of cells: the state of charge; the cell capacity; and the resistance.
According to one embodiment, the method comprises generating an indication of a fault in the event that the result of said measurement deviates from an expected value. Consequently, the fault detection can be made according to the above-mentioned principles in order so solve the problems related to prior art.
According to a further embodiment of the invention, it relates to a battery management system for monitoring the status of a plurality of connected battery cells in a battery pack, comprising: a plurality of battery cells which are arranged in at least two groups of cells; a sensor unit connected to said groups, said sensor unit being configured for providing a measurement of at least one parameter indicative of the state of operation of said battery pack; and a control unit connected to said sensor unit. According to said embodiment, the system further comprises: an arrangement of said groups of cells wherein: at least two of said groups comprise two or more cells, and at least two of said groups overlap so that a cell forms part of said overlapping groups; and a connection between said sensor unit and said groups; wherein the number of groups is less than the number of cells.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.